Comparative toxicity of salts to microbial processes in soil.

Abstract:
Soil salinization is a growing threat to global agriculture and carbon (C) sequestration, but to date it remains unclear how microbial processes will respond. We studied the acute response to salt exposure of a range of anabolic and catabolic microbial processes, including bacterial (leucine incorporation) and fungal (acetate incorporation into ergosterol) growth rates, respiration and gross N mineralization and nitrification rates. To distinguish effects of specific ions from those of overall ionic strength, we compared the addition of four salts frequently associated with soil salinization (NaCl, KCl, Na2SO4, K2SO4) to a non-saline soil. To compare the tolerance of different microbial processes to salt, and to interrelate the toxicity of different salts, concentration-response relationships were established. Growth-based measurements revealed that fungi were more resistant to salt exposure than bacteria. Effects by salt on C and N mineralization were indistinguishable and, in contrast with previous studies, nitrification was not found to be more sensitive to salt exposure than other microbial processes. Ion specific toxicity of certain salts could only be observed for respiration, which was less inhibited by salts containing SO4 (2-) than Cl(-) salts, in contrast with the microbial growth assessments. This suggested that the inhibition of microbial growth was solely explained by total ionic strength, while ionic specific toxicity should also be considered for effects on microbial decomposition. This difference resulted in an apparent reduction of microbial growth efficiency in response to exposure to SO4 (2-) salts but not to Cl(-) salts; no evidence was found to distinguish K(+) and Na(+) salts.